Semiconductor die assemblies with heat sink and associated systems and methods

ABSTRACT

Methods for forming semiconductor die assemblies with heat transfer features are disclosed herein. In some embodiments, the methods comprise providing a wafer having a first side and a second side opposite the first side, attaching a semiconductor die stack to the first side of the wafer, and forming a plurality of heat transfer features at the second side of the wafer. The heat transfer features can be defined by a plurality of grooves that define an exposed continuous surface of the wafer at the second side compared to a planar surface of the wafer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/655,736, filed Jul. 20, 2017; which is a continuation of U.S.application Ser. No. 15/134,788, filed Apr. 21, 2016, now U.S. Pat. No.9,716,019; which is a divisional of U.S. application Ser. No.14/451,192, filed Aug. 4, 2014, now U.S. Pat. No. 9,349,670; each ofwhich is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The disclosed embodiments relate to semiconductor die assemblies and tomanaging heat within such assemblies. In particular, the presenttechnology relates to stacked semiconductor die assemblies with heatsinks and associated systems and methods.

BACKGROUND

Packaged semiconductor dies, including memory chips, microprocessorchips, and imager chips, typically include a semiconductor die mountedon a substrate and encased in a plastic protective covering. The dieincludes functional features, such as memory cells, processor circuits,and imager devices, as well as bond pads electrically connected to thefunctional features. The bond pads can be electrically connected toterminals outside the protective covering to allow the die to beconnected to higher level circuitry.

Semiconductor manufacturers continually reduce the size of die packagesto fit within the space constraints of electronic devices, while alsoincreasing the functional capacity of each package to meet operatingparameters. One approach for increasing the processing power of asemiconductor package without substantially increasing the surface areacovered by the package (i.e., the package's “footprint”) is tovertically stack multiple semiconductor dies on top of one another in asingle package. The dies in such vertically-stacked packages can beinterconnected by electrically coupling the bond pads of the individualdies with the bond pads of adjacent dies using through-substrate vias(TSVs).

In vertically stacked packages, the heat generated is difficult todissipate, which increases the operating temperatures of the individualdies, the junctions therebetween, and the package as a whole. This cancause the stacked dies to reach temperatures above their maximumoperating temperatures (T_(max)) in many types of devices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a semiconductor die assemblyconfigured in accordance with an embodiment of the present technology.

FIGS. 2A-2E are cross-sectional views illustrating a portion of asemiconductor device at various stages in a method for makingsemiconductor die assemblies in accordance with selected embodiments ofthe present technology.

FIG. 3 is a cross-sectional view of a semiconductor die assemblyconfigured in accordance with another embodiment of the presenttechnology.

FIGS. 4A-4D are cross-sectional views illustrating a portion of asemiconductor device at various stages in a method for makingsemiconductor die assemblies in accordance with other selectedembodiments of the present technology.

FIG. 5 is a schematic view of a system that includes a semiconductor dieassembly configured in accordance with embodiments of the presenttechnology.

DETAILED DESCRIPTION

Specific details of several embodiments of stacked semiconductor dieassemblies having heat sinks and associated systems and methods aredescribed below. The terms “semiconductor device” and “semiconductordie” generally refer to a solid-state device that includes semiconductormaterial, such as a logic device, memory device, or other semiconductorcircuit, component, etc. Also, the terms “semiconductor device” and“semiconductor die” can refer to a finished device or to an assembly orother structure at various stages of processing before becoming afinished device. Depending upon the context in which it is used, theterm “substrate” can refer to a wafer-level substrate or to asingulated, die-level substrate. A person skilled in the relevant artwill recognize that suitable steps of the methods described herein canbe performed at the wafer level or at the die level. Furthermore, unlessthe context indicates otherwise, structures disclosed herein can beformed using conventional semiconductor-manufacturing techniques.Materials can be deposited, for example, using chemical vapordeposition, physical vapor deposition, atomic layer deposition, spincoating, and/or other suitable techniques. Similarly, materials can beremoved, for example, using plasma etching, wet etching,chemical-mechanical planarization, or other suitable techniques. Aperson skilled in the relevant art will also understand that thetechnology may have additional embodiments, and that the technology maybe practiced without several of the details of the embodiments describedbelow with reference to FIGS. 1-5.

As used herein, the terms “vertical,” “lateral,” “upper” and “lower” canrefer to relative directions or positions of features in thesemiconductor die assemblies in view of the orientation shown in theFigures. For example, “upper” or “uppermost” can refer to a featurepositioned closer to the top of a page than another feature. Theseterms, however, should be construed broadly to include semiconductordevices having other orientations, such as being inverted.

FIG. 1 is a cross-sectional view of a semiconductor die assembly 100(“assembly 100”) configured in accordance with an embodiment of thepresent technology. The assembly 100 includes a stack of firstsemiconductor dies 102 a carried by a second semiconductor die 102 b(collectively “semiconductor dies 102”). The second semiconductor die102 b, in turn, is carried by an interposer 120. The interposer 120 caninclude, for example, a semiconductor die, a dielectric spacer, and/oranother suitable substrate having electrical connectors (not shown),such as vias, metal traces, etc.) connected between the interposer 120and a package substrate 125. The package substrate 125 can include, forexample, an interposer, a printed circuit board, another logic die, oranother suitable substrate connected to electrical connectors 128 (e.g.,solder balls) that electrically couple the assembly 100 to externalcircuitry (not shown). In some embodiments, the package substrate 125and/or the interposer 120 can be configured differently. For example, insome embodiments the interposer 120 can be omitted and the secondsemiconductor die 102 b can be directly connected to the packagesubstrate 125.

The semiconductor dies 102 each include a plurality of vias 110 (e.g.,TSVs) that have a thermally and/or electrically conductive materialextending through the semiconductor dies 102. The vias 110 are alignedon one or both sides with corresponding electrically conductive elements112 between the semiconductor dies 102. In addition to electricalcommunication, the electrically conductive elements 112 can function asthermally conductive elements, or thermal conduits, through which heatcan be transferred away from the semiconductor dies 102 (as shown, e.g.,by arrow T₁). In some embodiments, the assembly 100 can also include aplurality of thermally conductive elements 113 (shown in broken lines)positioned interstitially between the electrically conductive elements112 in the space between adjacent semiconductor dies 102. The individualthermally conductive elements 113 can be at least generally similar instructure and composition as that of the electrically conductiveelements 112 (e.g., copper pillars). However, the thermally conductiveelements 113 are not electrically coupled to the semiconductor dies 102.Instead, the thermally conductive elements 113 can serve as additionalthermal conduits through which thermal energy can be transferred awayfrom the semiconductor dies 102 to transfer additional heat.

The semiconductor dies 102 can be at least partially encapsulated in adielectric underfill material 115. The underfill material 115 can bedeposited or otherwise formed around and/or between the semiconductordies 102 to electrically isolate the electrically conductive elements112 and/or enhance the mechanical connection between the semiconductordies 102. In some embodiments, the underfill material 115 can beselected based on its thermal conductivity to enhance heat dissipationthrough the semiconductor dies 102.

The semiconductor dies 102 can each be formed from a semiconductorsubstrate, such as silicon, silicon-on-insulator, compound semiconductor(e.g., Gallium Nitride), or other suitable substrate materials. Thesemiconductor substrate can be cut or singulated into semiconductor dieshaving any of variety of integrate circuit components or functionalfeatures, such as dynamic random-access memory (DRAM), staticrandom-access memory (SRAM), flash memory, other forms of integratedcircuit devices, including memory, processing circuits, imagingcomponents, and/or other semiconductor devices. In selected embodiments,the assembly 100 can be configured as a hybrid memory cube (HMC) inwhich the first semiconductor dies 102 a provide data storage (e.g.,DRAM dies) and the second semiconductor die 102 b provides memorycontrol (e.g., DRAM control) within the HMC. In some embodiments, theassembly 100 can include other semiconductor dies in addition to and/orin lieu of one or more of the semiconductor dies 102. For example, suchsemiconductor dies can include integrated circuit components other thandata storage and/or memory control components. Further, although theassembly 100 includes six dies stacked on the interposer 120, in otherembodiments the assembly 100 can include fewer than six dies (e.g., twodies, three dies, four dies, or five dies) or more than six dies (e.g.,eight dies, twelve dies, sixteen dies, thirty-two dies, etc.). Forexample, in one embodiment, the assembly 100 can include seven memorydies stacked on two logic dies.

As further shown in FIG. 1, the assembly 100 also includes a heat sink130 and an encapsulant or mold material 132 (e.g., an epoxy moldcompound) surrounding the first semiconductor dies 102 a. The heat sink130 is adjacent the mold material 132 and includes an exposed surface133 and a plurality of heat transfer features 135 along the exposedsurface 133. In one aspect of the illustrated embodiment of FIG. 1, theheat transfer features 135 define a plurality of recesses or grooves 136in the heat sink 130 that form fins which increase the surface area ofthe exposed surface 133 compared to a planar surface. In anotherembodiment, the heat transfer features 135 can be projections, such asfins, that extend away from the semiconductor dies. One advantage of theadditional surface area is that it permits a heat transfer medium, suchas air, to have increased thermal contact with the heat sink 130.Another advantage of the larger surface area is that it increases therate at which heat can be transferred or dissipated away from thesemiconductor dies 102. A related advantage is that the improved heatdissipation can lower the operating temperatures of the individualsemiconductor dies 102 such that they stay below their designatedmaximum temperatures (T_(max)). This, in turn, allows the semiconductordie assembly 100 to be more closely packed and smaller than aconventional die assembly.

The heat sink 130 can include crystalline, semi-crystalline, and/orceramic substrate materials, such as silicon, polysilicon, aluminumoxide (Al₂O₃), sapphire, and/or other suitable semiconductor materialshaving high thermal conductivities. In one embodiment described ingreater detail below, the heat sink 130 does not include IC devices norother active components, such as memory and logic circuitry. As such,the heat sink 130 does not provide any intermediary signal processing(e.g., logic operations, switching, etc.). Instead, the heat sink 130can be configured as a “blank die” or a “blank semiconductor substrate.”In various embodiments, the heat sink 130 can be similar in shape and/orsize as one or more of the semiconductor dies 102. For example, in theillustrated embodiment of FIG. 1, the heat sink 130 has the samefootprint as the first semiconductor dies 102 a, but has a smallerfootprint than the second semiconductor die 102 b. In another embodimentdescribed in greater detail below, the heat sink 130 can have a largerfootprint than all of the semiconductor dies in a semiconductor dieassembly.

FIGS. 2A-2E are cross-sectional views illustrating a portion of asemiconductor device 240 at various stages in a method for makingsemiconductor die assemblies in accordance with selected embodiments ofthe present technology. Referring first to FIG. 2A, the semiconductordevice 240 includes a semiconductor substrate, or semiconductor wafer250, containing a plurality of logic dies 252 separated from one anotherby dicing lanes 253. As shown, a plurality of memory dies 254(identified individually as first through fifth memory dies 254 a-e) isstacked upon each of the corresponding logic dies 252. The first memorydies 254 a include a plurality of contact pads 216 coupled tocorresponding contact pads 218 of the logic dies 252 by conductiveelements 212. After attaching the first memory dies 254 a, the secondthrough fifth memory dies 202 b-202 e can be stacked in sequence uponcorresponding first memory dies 254 a. Contact pads 214 of the secondthrough fifth memory dies 254 b-254 d can be connected to conductiveelements as disposed between each of the memory dies 254 (shown as smallconnective bumps in FIG. 2B). In an alternate embodiment, the entirestack of memory dies 254 can be preassembled, and the entire stack ofthe memory dies 254 can be attached to the corresponding logic dies 252at the same time.

FIG. 2B shows the semiconductor device 240 after stacking semiconductorsubstrates, or semiconductor blanks 230 (e.g., blank silicon dies), oncorresponding fifth memory dies 254 e and flowing an underfill material215 between each of the memory dies 254 and between the first memorydies 254 a and the logic dies 252. As shown, the memory dies 254 and thesemiconductor blanks 230 form individual die stacks 260 that areseparated from one another by gaps g₁. In the illustrated embodiment ofFIG. 2B, an interface material 257 is disposed between the semiconductorblanks 230 and the fifth memory dies 254 e. In various embodiments, theinterface material 257 can be made from what are known in the art as“thermal interface materials” (“TIMs”), designed to increase the thermalconductance at surface junctions (e.g., between a die surface and a heatspreader). TIMs can include silicone-based greases, gels, or adhesivesthat are doped with conductive materials (e.g., carbon nano-tubes,solder materials, diamond-like carbon (DLC), etc.), as well asphase-change materials.

FIG. 2C shows the semiconductor device 240 after encapsulating the diestacks 260 with a mold material 232. The mold material 232 can be heatedand compressed such that it liquefies and flows through the individualgaps g₁. After the mold material 232 fills the gaps g₁, it can beallowed to cool and harden. Once hardened, the mold material 232 can bethinned (e.g., via backgrinding) from a first thickness level L₁ to asecond thickness level L₂ to expose a first surface 233 a (e.g., aback-side surface) of each of the semiconductor blanks 230. In severalembodiments, the mold material 232 can be thinned until a portion 265 ofeach of the semiconductor blanks 230 projects beyond the mold material232 by a distance z₁, such as a distance of approximately 10 μm toapproximately 100 μm.

FIG. 2D shows the semiconductor device 240 after forming heat transferfeatures 235 in the first surface 233 a of each of the semiconductorblanks 230. In one embodiment, the heat transfer features 265 can beformed by cutting recesses or grooves 236 (e.g., via a dicing blade)into the semiconductor blanks 230. For example, a dicing blade can cutgrooves 260 through multiple semiconductor blanks in a semiconductorwafer before wafer dicing or simultaneously with wafer dicing. In someembodiments, other suitable processes, such as etching, can be used inaddition to or in lieu of mechanically cutting grooves.

Referring to the inset view of FIG. 2D, each of the heat transferfeatures 235 includes a first heat transfer wall 268, a second heattransfer wall 269, and a portion of the first surface 233a. In a furtheraspect of this embodiment, the first and second heat transfer walls 268and 269 can be generally parallel, and each wall can extend from thefirst surface 233 a to a recessed surface 270. In other embodiments,however, the heat transfer walls 268 and 269 can be non-parallel and/orextent completely through the semiconductor blank 230 to expose portionsof the interface material 257 aligned with the grooves 236.

In yet another aspect the illustrated embodiment of FIG. 2D, each heattransfer feature 235 has a generally similar shape and size. Forexample, the heat transfer feature 235 can be disposed in a portion ofsemiconductor substrate having a thickness ti defined by a distancebetween the first surface 233 a and a second surface 233 b (e.g., afront-side surface). Each heat transfer feature 235 can have a depth d₁and a first width w₁, and can be spaced apart from adjacent heattransfer features 235 by a second distance w₂. In one embodiment, d₁ isapproximately 150 and t₁ is approximately 300 μm. In another embodiment,d₁ is from about one-fifth to about three-fourths the value of t₁. Instill a further embodiment, d₁ is from about one-third to about one-halfthe value of t₁. In additional embodiments, d₁, t₁, w₁, and w₂ can haveother values depending on the heat transfer requirements.

FIG. 2E shows the semiconductor device 240 after it has been singulatedinto separate semiconductor die assemblies 200. As shown, thesemiconductor substrate 250 can be cut together with the mold material232 at the dicing lanes 253 (FIG. 2A) to singulate the logic dies 252and to separate the semiconductor die assemblies 200 from one another.Once singulated, the individual semiconductor die assemblies 200 can beattached to a substrate, such as a package or interposer substrate (notshown), at a subsequent processing stage. For example, in theillustrated embodiment the logic dies 252 include contact pads 219 thatcan be bonded to corresponding contact pads of the interposer 120 (FIG.1).

FIG. 3 is a cross-sectional view of a semiconductor die assembly 300(“assembly 300”) configured in accordance with another embodiment of thepresent technology. The assembly 300 can include features generallysimilar in structure and function to those of the semiconductor dieassemblies described in detail above. For example, the assembly 300 caninclude a stack of first semiconductor dies 302 a carried by a secondsemiconductor die 302 b (collectively “semiconductor dies 302”). Theassembly 300 also includes a mold material 332 and a heat sink 330having a plurality of heat transfer features 335 integrally formed in anexposed surface 333. In the illustrated embodiment of FIG. 3, however,the mold material 332 surrounds the heat sink 330 and all of thesemiconductor dies 302. Further, a peripheral portion 339 of the heatsink 330 extends beyond the footprint of the semiconductor dies 302. Inone aspect of this embodiment, the peripheral portion 339 can facilitateadditional heat transfer toward the periphery dies 302.

FIGS. 4A-4D are cross-sectional views illustrating a portion of asemiconductor device 440 at various stages in a method for makingsemiconductor die assemblies in accordance with other selectedembodiments of the present technology. Referring first to FIG. 4A, thesemiconductor device 440 includes a plurality of die stacks 460 formedon a semiconductor substrate, or semiconductor wafer 450 (e.g., a blanksilicon substrate). The die stacks 460 include a logic die 452 stackedon a plurality of memory dies 454 (identified individually as firstthrough fifth memory dies 454 a-e). The memory dies 454, in turn, arestacked on the semiconductor wafer 450. In the illustrated embodiment ofFIG. 4A, a plurality of recesses or first grooves 457 have been cut intothe semiconductor wafer 450 (e.g., with a dicing blade) between theindividual die stacks 460. As described in greater detail below, thefirst grooves 457 can be configured to facilitate the singulation ofsemiconductor dies from the semiconductor wafer 450. In otherembodiments, however, the first grooves 457 can be omitted.

FIG. 4B shows the semiconductor device 440 after encapsulating the diestacks 460 with a mold material 432. Similar to the mold material 232described above with reference to FIG. 2C, the mold material 432 can bedisposed between the die stacks 460 and subsequently thinned. Forexample, the mold material 432 can be thinned to expose contact pads 419of the logic die 452.

FIG. 4C shows the semiconductor device 440 after thinning thesemiconductor wafer 450 to form individual heat sinks 430 under the diestacks 460 and forming heat transfer features 435 in the heat sinks 430.In the illustrated embodiment of FIG. 4C, the semiconductor wafer 450has been thinned (e.g., via backgrinding) from a first a first substratelevel S₁ to second substrate level S₂ such that a portion of the moldmaterial 432 within the first grooves 457 is exposed to thereby separatethe portions of the semiconductor wafer 450 that define the heat sinks430 from each other. Once the semiconductor wafer 450 has been thinned,the heat transfer features 435 can be formed by cutting recesses orsecond grooves 436 into the heat sinks 430.

FIG. 4D shows the semiconductor device 440 after singulating thesemiconductor wafer 450 (FIG. 4C) into separate individual dieassemblies 400 from each other. As shown, the mold material 432 can becut at the first grooves 457 to singulate the heat sinks 430 from thesemiconductor wafer 450 and to separate the semiconductor die assemblies400 from one another. Similar to the die assemblies 200 described abovewith reference to FIG. 2E, the individual die assemblies 400 can beattached to a substrate, such as a package or interposer substrate (notshown), at a subsequent processing stage.

Any one of the interconnect structures and/or semiconductor dieassemblies described above with reference to FIGS. 1-4D can beincorporated into any of a myriad of larger and/or more complex systems,a representative example of which is system 590 shown schematically inFIG. 5. The system 590 can include a semiconductor die assembly 500, apower source 592, a driver 594, a processor 596, and/or other subsystemsor components 598. The semiconductor die assembly 500 can includefeatures generally similar to those of the stacked semiconductor dieassemblies described above, and can therefore include various featuresthat enhance heat dissipation. The resulting system 590 can perform anyof a wide variety of functions, such as memory storage, data processing,and/or other suitable functions. Accordingly, representative systems 590can include, without limitation, hand-held devices (e.g., mobile phones,tablets, digital readers, and digital audio players), computers, andappliances. Components of the system 590 may be housed in a single unitor distributed over multiple, interconnected units (e.g., through acommunications network). The components of the system 590 can alsoinclude remote devices and any of a wide variety of computer readablemedia.

From the foregoing, it will be appreciated that specific embodiments ofthe technology have been described herein for purposes of illustration,but that various modifications may be made without deviating from thedisclosure. For example, while described as blank dies or wafers in theillustrated embodiments, the wafer 450 and the dies 130, 230, 330, and450 can include memory and other functional features in otherembodiments. In such embodiments, these wafers and dies may be non-TSVdies that are generally thicker to accommodate heat transfer features.Further, although several of the embodiments of the semiconductor diesassemblies are described with respect to HMCs, in other embodiments thesemiconductor die assemblies can be configured as other memory devicesor other types of stacked die assemblies. In addition, while in theillustrated embodiments certain features or components have been shownas having certain arrangements or configurations, other arrangements andconfigurations are possible. Moreover, although advantages associatedwith certain embodiments of the new technology have been described inthe context of those embodiments, other embodiments may also exhibitsuch advantages and not all embodiments need necessarily exhibit suchadvantages to fall within the scope of the technology. Accordingly, thedisclosure and associated technology can encompass other embodiments notexpressly shown or described herein.

I/we claim:
 1. A method of manufacturing a semiconductor die assembly,comprising: providing a wafer having a first side and a second sideopposite the first side; attaching a semiconductor die stack to thefirst side of the wafer; and forming a plurality of heat transferfeatures at the second side of the wafer, wherein the heat transferfeatures are defined by a plurality of grooves that define an exposedcontinuous surface of the wafer at the second side compared to a planarsurface of the wafer.
 2. The method of claim 1, further comprising atleast partially encapsulating the semiconductor die stack with a moldmaterial.
 3. The method of claim 2, further comprising cutting throughthe semiconductor wafer and the mold material to singulate thesemiconductor die stack.
 4. The method of claim 1 wherein the pluralityof heat transfer features extend along a first width of thesemiconductor wafer, and wherein the semiconductor die stack includes asecond width less than the first width.
 5. The method of claim 1,further comprising, before forming the plurality of heat transferfeatures, disposing a mold material over the semiconductor die stack andthe first side of the wafer.
 6. The method of claim 1 wherein formingthe plurality of heat transfer features includes removing material fromthe wafer at the second side.
 7. The method of claim 1, furthercomprising, before forming the plurality of heat transfer features,thinning the wafer at the second side.
 8. The method of claim 1 whereinthe heat transfer features are first heat transfer features, the groovesare first grooves, and the semiconductor die stack includes an outermostsurface, the method further comprising: forming a plurality of secondheat transfer features along a semiconductor blank attached to theoutermost surface of the semiconductor die stack, wherein the secondheat transfer features are defined by a plurality of second grooves thatincrease an exposed continuous surface of the semiconductor blankcompared to a planar surface of the semiconductor blank.
 9. The methodof claim 8, further comprising encapsulating the second heat transferfeatures in a mold material.
 10. The method of claim 1, wherein thesemiconductor wafer includes peripheral portions that extend beyond afootprint of the semiconductor die stack.
 11. A method of manufacturinga semiconductor die assembly, comprising: providing a wafer having afirst side, a second side opposite the first side, and a plurality offirst grooves extending from the first side to an intermediate depth ofthe wafer; attaching a plurality of semiconductor die stacks to thefirst side of the wafer, wherein individual semiconductor die stacks aredisposed between adjacent first grooves; and forming a plurality of heattransfer features at the second side of the wafer, wherein the heattransfer features are defined by a plurality of second grooves thatdefine an exposed continuous surface of the wafer at the second sidecompared to a planar surface of the wafer.
 12. The method of claim 11,further comprising, before forming the plurality of heat transferfeatures, thinning the wafer at the second side at least to theintermediate depth.
 13. The method of claim 12 wherein thinning thewafer comprises singulating the individual semiconductor die stacks fromone another.
 14. The method of claim 12, further comprising, beforethinning the wafer, disposing a mold material over the first side of thewafer such that the semiconductor die stacks are encapsulated by themold material.
 15. The method of claim 14, further comprisingsingulating the individual semiconductor die stacks from one another bycutting through the mold material and the semiconductor wafer.
 16. Themethod of claim 11, further comprising at least partially encapsulatingthe semiconductor die stack with a mold material.
 17. The method ofclaim 16, further comprising cutting through the semiconductor wafer andthe mold material to singulate a semiconductor device including theindividual semiconductor die stack and the cut semiconductor wafer. 18.The method of claim 11 wherein the heat transfer features are first heattransfer features and the individual semiconductor die stacks eachinclude an outermost surface, the method further comprising: forming aplurality of second heat transfer features along a semiconductor blankattached to the outermost surface of the semiconductor die stack,wherein the second heat transfer features are defined by a plurality ofthird grooves that increase an exposed continuous surface of thesemiconductor blank compared to a planar surface of the semiconductorblank.
 19. The method of claim 11 wherein the semiconductor waferincludes a first thickness, and wherein forming the plurality of heattransfer features includes removing material from the second side of thewafer such that the heat transfer features extend a first distancethrough the wafer, the first distance being less than the firstthickness.
 20. The method of claim 11 wherein the individualsemiconductor die stacks each include a plurality of semiconductor diesstacked in a first direction away from the semiconductor wafer, andwherein the heat transfer features face toward a second directionopposite the first direction.